The present invention relates to phase locked loops (PLLs) and, in particular, to lock detectors for detecting when the frequency and phase of an output clock signal generated by the PLL matches that of an input clock signal.
A Phase Locked Loop (PLL) is a control system that provides an output signal which is phase and frequency synchronized with an input signal. PLL circuits are used in numerous circuit applications, such as frequency synthesizers, clock generation, clock recovery and the like. A PLL may be implemented in either a digital or analog form. However, PLLs are often implemented as Silicon on Chip type devices (SoCs) that include on-chip clocks for generating the various signals. An integrated PLL on the chip avoids the need to provide a separate clock generator circuit and thus provides improved accuracy clocks and jitter performance.
Generally speaking, a PLL typically includes a phase detector and an oscillator. The phase detector receives the input signal (hereinafter “the reference signal”) and a feedback signal provided via the oscillator, and generates an output signal proportional to the phase difference between the input and reference signals. If a phase or frequency difference exists, the oscillator is controlled to modify the frequency of its output signal, and thus the feedback signal, to correct the frequency and phase difference. Alternatively, in the event that the reference signal and the feedback signal are synchronous in frequency and phase, the oscillator maintains the phase and frequency of its output signal, and thus the feedback signal.
If the frequency and phase of the reference signal is synchronized with the feedback signal the PLL is in a “locked condition”. A PLL is considered to be “locked” when the reference and feedback signals are matched within a predetermined phase and frequency limit. On the other hand, if the phase and frequency of the reference and feedback signals differ by more than the predetermined limit, the PLL is “out of lock”.
Typically, a PLL includes a lock detector circuit which provides lock status signals, usually in the form of “lock” and “out-of-lock” status signals, which provide an indication of the PLL lock status. Thus, a lock detector circuit typically asserts a “lock” status signal when the difference between the reference signal and the feedback signal are within the phase and frequency limit, and de-asserts the lock status signal otherwise.
In many applications, the status signals provide a critical role in system operation and so it is desirable that a lock detector provide robust and reliable performance, which typically involves low false indications, as may be caused by signal jitter. For reliable operation, a lock detector circuit must be able to finely resolve phase differences between the reference and feedback signals.
In addition, delays in generating the status signals can affect the overall timing performance of the PLL, to the extent that a lock detector can introduce propagation delays which contribute to reduced system responsiveness. Hence, ideally a lock detector should provide robust performance and minimum propagation delay.
Some prior art lock detectors rely on mechanisms that count cycles of the feedback and reference clock cycles and then compare respective counter values using a synchronized comparator. In some lock detectors, the synchronized comparator compares the counter values on the active edge of one of the clock signals, in other words either the reference or feedback signal, and provides an output signal which is synchronized with that clock. Synchronizing the comparator with one of the clock signals may introduce undesirable phase match/mismatch information propagation since it prevents the lock detector from sensing phase differences that may arise between the active edges of the synchronizing clock signal.
It would be advantageous to have a lock detector with improved phase difference sensitivity. This may be important for devices that require accurate clocks such as devices for protocols such as for USB, PCI Express, and DDR. It also would be advantageous to have a lock detector that reduces the likelihood of false activations and deactivations of PLL lock status signals and thus provide improved robustness and reliability.